Psychiatric disorders are pathological conditions of the brain characterized by identifiable symptoms that result in abnormalities in cognition, emotion, mood, or affect. These disorders may vary in severity of symptoms, duration, and functional impairment. Psychiatric disorders afflict millions of people worldwide resulting in tremendous human suffering and economic burden due to lost productivity and dependent care.
Over the past several decades, the use of pharmacological agents to treat psychiatric disorders has greatly increased, largely due to research advances in both neuroscience and molecular biology. In addition, chemists have become increasingly sophisticated at creating chemical compounds that are more effective therapeutic agents with fewer side effects, targeted to correct the biochemical alterations that accompany mental disorders.
Yet, despite the many advances that have occurred, many psychiatric diseases remain untreated or inadequately treated with current pharmaceutical agents. In addition, many of the current agents interact with molecular targets not involved with the psychiatric disease. This indiscriminate binding can result in side effects that can greatly influence the overall outcome of therapy. In some cases the side effects are so severe that discontinuation of therapy is required.
Depression is an affective disorder, the pathogenesis of which cannot be explained by any single cause or theory. It is characterized by a persistently low mood or diminished interests in one's surroundings, accompanied by at least several of the following symptoms: reduced energy and motivation, difficulty concentrating, altered sleep and appetite, and at times, suicidal ideation (American Psychiatric Association: Diagnostic and Statistical Manual of Mental Disorders, ed. 4. Washington, American Psychiatric Association, 1994). Major depression is associated with high rates of morbidity and mortality, with suicide rates of 10-25% (Kaplan H I, Sadock B J (eds): Synopsis of Psychiatry. Baltimore, Williams & Wilkins, 1998, p. 866).
Depression is believed to result from dysfunction in the noradrenergic or serotonergic systems, more specifically, from a deficiency of certain neurotransmitters (NTs) at functionally important adrenergic or serotonergic receptors.
Neurotransmitters produce their effects as a consequence of interactions with specific receptors. Neurotransmitters, including norepinephrine (NE) and/or serotonin (5-hydroxytryptamine, or 5-HT), are synthesized in brain neurons and stored in vesicles. Upon a nerve impulse, NTs are released into the synaptic cleft, where they interact with various postsynaptic receptors. Regional deficiencies in the synaptic levels of 5-HT and/or NE are believed to be involved in the etiology of depression, wakefulness, and attention.
Norepinephrine is involved in regulating arousal, dreaming, and moods. Norepinephrine can also contribute to the regulation of blood pressure, by constricting blood vessels and increasing heart rate.
Serotonin (5-HT) is implicated in the etiology or treatment of various disorders. The most widely studied effects of 5-HT are those on the CNS. The functions of 5-HT are numerous and include control of appetite, sleep, memory and learning, temperature regulation, mood, behavior (including sexual and hallucinogenic behavior), cardiovascular function, smooth muscle contraction, and endocrine regulation. Peripherally, 5-HT appears to play a major role in platelet homeostasis and motility of the GI tract. The actions of 5-HT are terminated by three major mechanisms: diffusion; metabolism; and reuptake. The major mechanism by which the action of 5-HT is terminated is by reuptake through presynaptic membranes. After 5-HT acts on its various postsynaptic receptors, it is removed from the synaptic cleft back into the nerve terminal through an uptake mechanism involving a specific membrane transporter in a manner similar to that of other biogenic amines. Agents that selectively inhibit this uptake increase the concentration of 5-HT at the postsynaptic receptors and have been found to be useful in treating various psychiatric disorders, particularly depression.
Approaches to the treatment of depression over the years have involved the use of agents that increase the levels of NE and 5-HT, either by inhibiting their metabolism (e.g., monoamine oxidase inhibitors) or reuptake (e.g., tricyclic antidepressants or selective serotonin reuptake inhibitors (SSRIs)).
There are more than twenty (20) approved antidepressant drugs available in the United States. The classical tricyclic antidepressants (TCAs) currently available block primarily the uptake of NE and also, to varying degrees, the uptake of 5-HT, depending on whether they are secondary or tertiary amines. Tertiary amines such as imipramine and amitriptyline are more selective inhibitors of the uptake of 5-HT than of catecholamines, compared with secondary amines such as desipramine.
Selective serotonin reuptake inhibitors have been investigated as potential antidepressants. Fluoxetine (PROZAC®), sertraline (ZOLOFT®), and paroxetine (PAXIL®) are three examples of SSRIs currently on the U.S. market. These agents do not appear to possess greater efficacy than the TCAs, nor do they generally possess a faster onset of action; however, they do have the advantage of causing less side-effects. Of these three SSRIs, paroxetine is the most potent inhibitor of 5-HT uptake, fluoxetine the least. Sertaline is the most selective for 5-HT versus NE uptake, fluoxetine the least selective. Fluoxetine and sertraline produce active metabolites, while paroxetine is metabolized to inactive metabolites. The SSRIs, in general, affect only the uptake of serotonin and display little or no affinity for various receptor systems including muscarinic, adrenergic, dopamine, and histamine receptors.
In addition to treating depression, several other potential therapeutic applications for SSRIs have been investigated. They include treatment of Alzheimer's disease, aggressive behavior, premenstrual syndrome, diabetic neuropathy, chronic pain, fibromyalgia, and alcohol abuse. For example, fluoxetine is approved for the treatment of obsessive-compulsive disorder (OCD). Of particular significance is the observation that 5-HT reduces food consumption by increasing meal-induced satiety and reducing hunger, without producing the behavioral effects of abuse liability associated with amphetamine-like drugs. Thus, there is interest in the use of SSRIs in the treatment of obesity.
Venlafaxine (EFFEXOR®) is a dual-reuptake antidepressant that differs from the classical TCAs and the SSRIs chemically and pharmacologically in that it acts as a potent inhibitor of both 5-HT and NE uptake. Neither venlafaxine nor its major metabolite have a significant affinity for adrenergic alpha-1 receptors. Venlafaxine possesses an efficacy equivalent to that of the TCAs, and a benign side effect profile similar to those of the SSRIs.
Dopamine is hypothesized to play a major role in psychosis and certain neurodegenerative diseases, such as Parkinson's disease, where a deficiency in dopaminergic neurons is believed to be the underlying pathology. Dopamine affects brain processes that control movement, emotional response, and ability to experience pleasure and pain. Regulation of DA plays a crucial role in our mental and physical health. Certain drugs increase DA concentrations by preventing DA reuptake, leaving more DA in the synapse. An example is methylphenidate (RITALIN®), used therapeutically to treat childhood hyperkinesias and symptoms of schizophrenia. Dopamine abnormalities are believed to underlie some of the core attentional abnormalities seen in acute schizophrenics.
A therapeutic lag is associated with the use of these drugs. Patients must take a drug for at least three (3) weeks before achieving clinically meaningful symptom relief. Furthermore, a significant number of patients do not respond to current therapies at all. For example, it is currently estimated that up to thirty percent (30%) of clinically diagnosed cases of depression are resistant to all forms of drug therapy.